1. Field of the Invention
The present invention relates to a system and process for the separation of liquid hydrocarbon gases into components by fractionation. More particularly the invention relates to adapting and interrelating the fractionating units of an LPG (liquified petroleum gas) fractionating system to obtain at least a portion of the energy to operate the system from within the system itself.
2. Related Art
Natural gasoline is derived by condensation of vapors from gas wells of normally liquid hydrocarbons representing light gasoline fractions. Gas containing such vapors is referred to as "wet gas" or "rich gas" to distinguish it from "dry gas" or "lean gas" which is essentially free from them. Removal of gasoline vapors from natural gas used for heating purposes is required also because of potential fire hazard.
Hydrocarbons present in natural gas are of the saturated type, methane having the lowest boiling point and existing in the vapor form under most conditions. The greater the number of carbon atoms in the molecule, the higher the boiling point. These hydrocarbons include ethane (C.sub.2 H.sub.6), propane (C.sub.3 H.sub.8), butane (C.sub.4 H.sub.10), pentane (C.sub.5 H.sub.12) and the heavier ones in decreasingly smaller quantities.
Ethane is a gas similar to methane. Propane, which is also a gas under normal conditions, however, may be liquefied by applying considerable pressure. Butane liquifies with still greater ease because its boiling point under normal atmospheric pressure is still higher, close to the freezing point of water.
Liquid hydrocarbons present in natural gas and boiling within the gasoline range (butanes, pentanes and heavier) are referred to under the name of "natural gasoline." Motor fuel sold at the service stations is a blend of various petroleum products, including natural gasoline or its equivalents. The presence of dissolved gases is undesirable if natural gasoline is used for the preparation of the motor gasoline. Separation of these gases from the liquid hydrocarbon portion is, therefore, very important in commercial practice. This separation is accomplished by the "stabilization processes" and the resulting gasoline is referred to as "stabilized gasoline."
Stabilization of gasoline consists in separating gaseous and liquid hydrocarbons by what is known as fractional distillation. Fractional distillation is a process by which the vapors given off are passed through columns containing numbers of plates of special design. The plates have narrow passages for gases and vapors flowing upward. These vapor passages in the plates can be any one or mixtures of vapor dispersion devices such as bubble caps, sieve trays, valve trays, etc. The overflow pipes or liquid downcomers are reserved for liquid traveling downward and may be of various geometric configurations. At each plate the vapors condense and the resulting liquid partially revaporizes. The newly formed vapors ascend to the next higher plate of the fractionating column, while the condensed vapors and unevaporated fraction flows downward to the next plate below. After each revaporization the ascending vapors become progressively richer in the low-boiling materials. The process thus permits separation of liquid hydrocarbon mixtures into groups that boil within narrow temperature ranges or into individual hydrocarbons, if their boiling points differ considerably from the rest of the hydrocarbons in a mixture.
Stabilization of gasoline results in the reduction of its vapor pressure, i.e., the pressure that is registered when gasoline is confined in a closed vessel under prescribed conditions of testing. The desired vapor pressure is obtained by varying the content of lighter hydrocarbons, for example, propane and butane. In general, propane is an undesirable constituent of gasoline in the engine in any proportions. If gasoline contains an excess of butane, this excess may be separated and then added to another gasoline deficient in butane; conversely, if the gasoline is deficient in butanes, pure components from another source may be blended back into the gasoline. When propane is removed, the process is referred to as the "depropanizing process" and the fractionating column in which this is accomplished is known as a "depropanizer." When butane is removed, the process is referred to as the "debutanizing process," and the equipment is known as a "debutanizer." Similarly a "deethanizer" is employed for equipment for separating ethane and a "depentanizer" for equipment for separating pentane, a hydrocarbon boiling at a higher temperature than butane, from the heavier portion of the natural gasoline.
Fractionation of LPG, e.g., raw natural gasoline (U.S. Pat. No. 3,055,826, Arnett) and hydrocarbon conversion products (U.S. Pat. No. 2,954,341, Stiles), to recover the components such as methane, ethane, propane, butane and gasoline is conventional, although there may be variations in the sequence of component removal, conditions and configuration of equipment used in such fractionations and other similar fractionations. For example, U.S. Pat. No. 4,460,396, Kaiser, et al. discloses recovering ethylene from a C.sub.2 stream using two columns with the overhead from the second column heat exchanged with the reboil of the first column; and U.S. Pat. No. 3,150,199, Greco, et al. discloses deethanizing and debutanizing followed by depropanizing hydrocarbon streams. Other fractionations have used an overhead from one column to heat the reboil of another column such as U.S. Pat. Nos. 2,916,888 (Cobb); 3,324,010 (Bauer, et al.); 4,256,541 (Muller, et al.); 4,372,822 (Muller, et al.); and 4,555,311 (Ward).
The same fractionation and heat utilization techniques also are employed in the separation of the various components of hydrocarbon mixtures from other sources. e.g., pyrolysis, cracking, hydroforming, isomerization, etc. These components can be saturated, unsaturated, aromatic or naphthenic. These components or cuts (mixtures of components) may, in addition to providing natural gasoline, provide raw materials for various petrochemicals, plastics, or synthetic rubber
It is an advantage of the present invention that the deethanizer is operated in a manner to provide more efficient operation and greater capacity than previously. It is a further advantage that the debutanizer is operated in a manner to recover all of the heat from the overhead vapors normally wasted and utilize it to provide some of the heat required for separation of other components of the fractionation system. It is a feature of the present invention that the deethanizer is operated at temperatures and pressure sufficiently low so that the waste heat from the debutanizer overhead provides a substantial a portion of the energy for the operation of the deethanizer reboiler. These and other advantages and features will become apparent from the following description.